Atheism is immoral
This argument can be seen as a futile attempt at an Ad hominem attack, and does not bring the argument anywhere. Atheists do not derive their Morality from Religious sources but that does not mean they are not moral whatsoever, it just means that any morality is derived from a source elsewhere (see Secular morality).
A Counter argument to this could easily be along the lines of; "well judging from the Bible, you're less moral than me".
All throughout the history of most religions, there have been numerous atrocities involving numerous deaths and that includes Christianity, see Christian morality. The statement made by the theist (presumably) in this case fails to realise this and when encountered with a response such as the above, they will most likely resort to weak rationalisations to justify both their belief and their statement.